The paradigm of using a "weak" linker reference to find what was
previously the vm_init_routine() doesn't work reliably on all compile
environments supported by the simulators. This has been reported
in #794 and it came up again in #862. This change assures that
it will not come up again AND it reliably solves the problem with
Visual Studio compilers (and linker) that randomly chooses whether
to have the desired effect or not.
Of the 82 simulators which are currently part of simh, only these
three used the sim_vm_init() interface, so removing it had relatively
minor impact.